The altered autonomic regulation of the cardiovascular system implicated in the increased incidence of cardiovascular diseases with age is likely related to attenuated arterial baroreflex sensitivity. Even in the absence of disease, aging alone reduces baroreflex-mediated cardiac vagal responses and baroreflex inhibition of vascular sympathetic outflow. Greater stiffness of barosensory areas associated with age-related declines in vascular compliance may be the primary culprit in altered baroreflex function with age. Moreover, greater fitness is associated with greater vascular compliance, and, thus may help to maintain baroreflex autonomic circulatory control with age. The proposed research will use unique vascular ultrasonography measurements to relate the stretch of a primary barosensory region, the carotid artery to baroreflex-mediated cardiovascular autonomic responses in young and active and inactive older humans. Thus, insight to the relations between carotid vascular stiffness and baroreflex gain with aging and fitness will be gained. Moreover, direct measurements of barosensory afferent activity in the carotid sinus nerve will be made during acute increases of arterial pressure in patients during endarterectomy surgery. This will provide a validation of ultrasound derived assessments of baroreflex gain against the relations between carotid arterial pressures and baroreceptive afferent activity. This novel approach to baroreflex physiology in humans will delineate not only the role of carotid stiffness in barosensory afferent responses to increases in arterial pressure, but also the role of exercise in reducing the effect of vascular stiffness on attenuated baroreflex function with age. Because the arterial baroreflex is the dominant factor which determines the mean level of cardiac vagal tone, these studies may suggest mechanisms to enhance vagal tone and decrease the risk of adverse outcomes with cardiovascular diseases.